Tangential fan with cutoff assembly and vibration control for electric discharge laser

ABSTRACT

A tangential fan and cutoff assembly for recirculating a lasing gas mixture has blade members, which vary in circumferential position stepwise from end to end, and/or a tapered anode assembly. The number of blade members can be constant or variable between ends. The circumferential position of blade members can shift monotonically or reversibly between ends. Blade members are stiffened by optimally selecting the number and placement of hub members to control the natural vibration frequency of the fan. Methods of forming tangential fans include casting, and machining from a solid block. Monolithic structures can be joined, typically by electron-beam welding. Casting, welding, and machining processes introduce no additional contaminants. Tangential fans produced have mechanical rigidity, accurate tolerances, and low contaminant concentrations. Blade members can be formed into air foil shapes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation In Part of U.S. application Ser. No.09/143,633, filed Aug. 28, 1998, entitled "Tangential Fan For ExcimerLaser", by Thomas Hofmann, et al. This application is also related toU.S. application Ser. No. 09/143,040, filed Aug. 28, 1998, entitled"Twisted Blade Tangential Fan For Excimer Laser," by Thomas Hofmann,commonly assigned. The specifications of the above applications areincorporated herein by reference in their entirety. This application isfurther related to U.S. application Ser. No 09/311,680 [Docket No.99-0014-01], entitled "Soldered Fan Assembly For Electric DischargeLaser," by Thomas Hofmann, James K. Howey, et al., commonly assigned andfiled herewith, the specification of which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to tangential fans, particularlytangential fans for producing gas flow in a gas laser chamber, and moreparticularly tangential fans for producing gas flow in excimer lasersand other pulse gas discharge lasers.

BACKGROUND

Transversely excited (TE) pulsed gas discharge lasers commonly include atangential fan to recirculate lasing gas inside a laser chamber. FIGS.1a and 1b are cross-sectional end and side views respectively showingthe inner structure of a laser chamber 100 in a conventional TE excimerlaser (see Akins et al., U.S. Pat. No. 4,959,840, issued Sep. 25, 1990,and incorporated herein by reference in its entirety). A laser enclosure102 provides isolation between a laser chamber interior 105 and theexterior 110. Typically enclosure 102 is formed by a pair of halfenclosure members 112 and 114 (see FIG. 1a), which are coupled togetherand sealed using an o-ring seal 116, extending along a perimeter ofenclosure 102. Laser chamber interior 105 is filled to a predeterminedpressure with a lasing gas 108. A pulsed gas discharge is generated in adischarge region 122 by a high voltage pulse applied between a cathodeassembly 118 and an anode assembly 120. The pulsed gas dischargetypically produces excited argon fluoride, krypton fluoride, or fluorinemolecules, which generate laser pulse output energy. The pulse outputenergy propagates from discharge region 122 through an optical outputwindow assembly 162 (see FIG. 1b). Cathode assembly 118 and anodeassembly 120, defining discharge region 122, extend parallel to oneanother along the length of laser chamber 100.

Recirculation of lasing gas 108 is provided by a tangential fan 140,which rotates about an axis 142 and includes a plurality ofsubstantially parallel straight blade members 144 extending along thelength of laser chamber 100 between hub members 146. A typical rotationrate for current tangential fans is on the order of approximately 3800revolutions per minute (rpm). As shown by arrows in FIG. 1a, the flow ofgas 108 is upward through tangential fan 140 and transversely acrossdischarge region 122 as directed by a vane member 152. Lasing gas 108that has flowed through discharge region 122 becomes dissociated andheated considerably by the pulsed gas discharge. A gas-to-liquid heatexchanger 158 (not shown in FIG. 1b) extending along the length of laserchamber 100 is positioned in the gas recirculation path to cool theheated gas. Other vane members, e.g. vane members 160, direct the flowof gas 108 through heat exchanger 158 and elsewhere along the gasrecirculation path. Recirculation cools and recombines lasing gas 108,thereby allowing repetitively pulsed laser operation without replacinglasing gas 108.

There are a variety of current issues relating to laser chamber 100 andits associated components, including, among other things, thosedescribed below.

The present tangential fan is difficult and expensive to fabricate.Blade members 144 and hub members 146 are individually stamped andformed from aluminum or another suitable alloy, such as analuminum/bronze alloy, then dip brazed together to form tangential fanassembly 140, using a braze material typically containing approximately13 per cent silicon by weight. This is a tedious and labor-intensiveprocess. Because the brazed fan assembly has poor mechanical rigidity,post-machining can cause damage and warpage and is thus difficult orimpractical. Therefore it is difficult to achieve precision alignmentand critical tolerances. The brazed tangential fan assembly 140 istypically coated with electroless nickel.

Since lasing gas 108 is recirculated and reused, it is important tomaintain cleanliness and to prevent contamination of the gas environmentwithin laser chamber interior 105, in order to maximize the pulse energyperformance, stability, and working life of lasing gas 108.

Undesirable vibrations in the rotating fan assembly adversely affectbearing life. Reduction of these vibrations will reduce bearing wear andallow the possibility of increasing the fan rotation speed for increasedgas flow velocity. Particularly, adverse vibrations are associated withthe low present natural vibrational frequency of the rotordynamicassembly, including the fan, bearings, shafts, and magnetic rotor. Thislow natural frequency is largely attributable to low first andsubsequent bending mode frequencies of the fan, due to poor mechanicalstiffness.

An aerodynamic buffeting effect has been observed, which, among otherthings, transmits vibrations to the fan bearings, causing bearing wearand premature failure. Measurements of the frequency of these vibrationssuggest that they are caused by gas pressure fluctuations generated eachtime a fan blade member 144 passes in close proximity to the edge ofanode assembly 120. Of importance, the clearance between fan blademembers 144 and the proximate edge of anode assembly 120 is particularlyclose, in order to minimize reverse flow leakage and maximize gas flowefficiency. Previous attempts to reduce aerodynamic buffeting byreshaping the anode assembly have resulted in an undesirable reductionin gas flow velocity by approximately ten or more per cent.

Many applications require a substantially constant laser pulse outputenergy. However, strong and undesirable fluctuations in pulse outputenergy have been observed. These fluctuations have been found to beparticularly severe at high laser pulse repetition rates.

Accordingly, it would be desirable to fabricate a tangential fanassembly economically, such that the finished fan assembly has improvedmechanical rigidity against vibrations. Additionally, it would bedesirable to minimize or eliminate potential contaminants from the laserchamber. Further, it would be desirable to minimize or eliminatevibrations arising from aerodynamic buffeting, and to minimize oreliminate pulse output energy fluctuations in a TE pulsed gas laser,particularly at high laser pulse repetition rates.

SUMMARY

A gas laser apparatus includes a tangential fan and cutoff assembly,configured to recirculate a lasing gas mixture. Generally, in accordancewith the invention, a blade member of the fan varies in circumferentialposition between a first end flange and a second end flange. In someembodiments, this variation in circumferential position is accomplishedin a stepwise offset fashion, with the tangential fan partitionedlongitudinally into sections. In some embodiments, an equivalent effectis achieved by tapering the edge of the anode or cutoff assembly.

In some configurations, circumferentially adjacent blade members arespaced evenly circumferentially relative to one another. Some versionshave an odd integral number of blade members around the circumference.In some versions the circumferential number of blade members is constantlongitudinally between the end flanges, whereas in other versions thecircumferential number of blade members is variable between the endflanges. In some configurations, the circumferential position of blademembers varies monotonically between the two end flanges. In otherconfigurations, the variation in circumferential position reversesdirection circumferentially one or more times between the two endflanges.

The tangential fan can operate in the chamber of a transverse-excitedexcimer laser, more particularly a krypton fluoride or argon fluorideexcimer laser, or of a fluorine (F₂) molecular gas laser.

Blade members extend longitudinally between and adjacent to the outsidecircumference of the end flanges. Typically, the blade members arestiffened by one or more transverse substantially annular hub members,parallel with and spaced between the end flanges. In particular, atangential fan according to embodiments of the invention is stiffened byoptimizing the number and placement of annular hub members, therebycontrolling the natural frequency of the bending vibrational mode of thefan. This reduces fan vibration without substantially changing any otherfan performance characteristic.

A tangential fan in accordance with the invention can be made using aconventional method of brazing together individually stamped and formedblade members and hub members. The invention provides improved methodsof forming a tangential fan, involving forming monolithic fan structuresby casting, or by machining out of a solid block of material. Thetangential fans formed have superior mechanical rigidity, more accuratetolerances, and lower contaminant concentrations than conventionallymade tangential fans. The methods provided by the invention are alsoapplicable to tangential fans having conventional geometry.

The monolithic structures formed in accordance with some embodimentsinclude integral hub and blade members. In some embodiments, two or moresuch monolithic structures are joined together, typically byelectron-beam welding. In the casting, welding, and machining process noadditional materials are introduced that might cause contamination, wheninstalled in the laser chamber. The methods provided by the inventionfacilitate forming of blade members into an air foil shape. In someembodiments, the inside diameter of the hub members is machined flushwith the inside diameter defined by the plurality of blade members.

Casting can be performed by die casting or investment casting. In someembodiments, an aluminum casting alloy containing additive metalsconsisting substantially of 3.5-6.5 per cent copper and 0-2.5 per centnickel or 0-1.5 per cent silver is used. For machined monolithicstructures, 6061 aluminum alloy is typically used. Finishing processestypically include post-machining, electropolishing, and electrolessnickel coating.

The present invention is better understood upon consideration of thedetailed description below, in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings. For simplicity and ease ofunderstanding, common numbering of elements within the illustrations isemployed where an element is the same in different drawings.

FIGS. 1a and 1b are cross-sectional end and side views respectivelyshowing the inner structure of a laser chamber in a conventional TEexcimer laser;

FIG. 2a is an isometric view of a portion of a tangential fan withoutcylindrical symmetry, in accordance with an embodiment of the presentinvention;

FIG. 2b is a graphical representation of the dependence ofcircumferential offset angle on sections of a tangential fan, inaccordance with an embodiment of the present invention;

FIG. 2c is an isometric view of a skewed or twisted substantiallyhelical blade fan structure, in accordance with an embodiment of thepresent invention;

FIGS. 2d and 2e are schematic end and front views respectively of atangential fan with anode assembly having a tapered anode support bar orcutoff;

FIG. 3a is an isometric view of a cast section of a tangential fanassembly, in accordance with one embodiment of the present invention;

FIG. 3b is an isometric view of a portion of a tangential fan assemblyincluding a plurality of sections joined together longitudinally, inaccordance with one embodiment of the present invention;

FIG. 3c is a schematic cross-sectional view of an airfoil blade memberviewed along direction C--C of FIG. 3a;

FIGS. 4a and 4b are respectively longitudinal and end views of amonolithic tangential fan structure machined from a single block of 6061aluminum alloy, in accordance with an embodiment of the presentinvention;

FIG. 4c is a detail view showing fillets connecting blade members with ahub member of a monolithic machined tangential fan structure;

FIGS. 4d and 4e are an end view and a cross-sectional side view,respectively, of a rotating shaft assembly configured to attach to atangential fan structure, in accordance with some embodiments;

FIG. 4f is a cross-sectional detail view of machined tangential fanstructure taken along direction F--F of FIG. 4a; and

FIG. 5 is a graphical representation of the dependence of relative laseroutput energy of an excimer laser on concentration for commoncontaminants.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following is a detailed description of illustrative embodiments ofthe present invention. As these embodiments of the present invention aredescribed with reference to the aforementioned drawings, variousmodifications or adaptations of the methods and or specific structuresdescribed may become apparent. These descriptions and drawings are notto be considered in a limiting sense as it is understood that thepresent invention is in no way limited to the embodiments illustrated.

Referring to FIGS. 1a and 1b, undesired fluctuations in laser pulseoutput energy have been found to originate from shock energy reflectedoff rotating blade members 144 of tangential fan 140. T. Hofmann et al.,"Origin of Energy Fluctuations at High Repetition Rate," CYMER INC.Technical Memo, Mar. 31, 1997, recites, among other things: "The blowerinteracts with the way shock waves from the discharge can re-enter thedischarge region. This can be done either by direct reflection off thefan blades or by creating an angle-dependent transmission for shockwaves traveling through the fan. A modulation with the blower speed onlyoccurs for PRFs where the time of flight coincides with a certain shockwave path. In any case, for PRFs below 2.5 kHz and the given chambergeometry it is obvious that shock waves undergo multiple reflectionsbefore re-entering the discharge." As used in the above quotation "PRF"is the standard abbreviation for "Pulse Repetition Frequency."

Pulsed gas discharges in discharge region 122 generate acoustic shockwaves in lasing gas 108, which propagate from discharge region 122through lasing gas 108 and are reflected from solid surfaces, e.g., heatexchanger 158 and inner walls of enclosure 102, inside laser chamber100. A portion of the reflected shock energy reenters discharge region122, where it interacts with the electrical and optical properties oflasing gas 108. This reflected shock energy interaction can increase ordecrease the pulse output energy of the laser.

Some reflecting surfaces, e.g., chamber walls, are stationary, whereassurfaces of tangential fan 140 are rotating with a substantially regularrotational period. Changes in laser pulse output energy due to shockenergy reflected from stationary surfaces are relatively uniform andtolerable, whereas pulse output energy changes due to shock energyreflected from rotating fan surfaces exhibit fluctuations.

Attempts to date to overcome this problem have met with limited success.Redesigning vane members 152, 160 in laser chamber 100 only partiallyreduces pulse output energy fluctuations. Applying acoustic dampingmaterial to surfaces within laser chamber 100 introduces porosity, whichprovides a source of contamination and also extends the passivation timeduring processing of laser chamber 100.

The structure of conventional tangential fan 140 incorporating straightparallel blade members 144 has cylindrical symmetry. This traditionalcylindrical symmetry promotes periodic reflection of shock energy, whichis reinforced in phase along the entire length of tangential fan 140.The present invention provides for a tangential fan structure thatbreaks the traditional cylindrical symmetry and minimizes reinforcedin-phase periodic reflection of shock energy, allowing a more uniformlaser pulse output energy.

FIG. 2a is an isometric view of a portion of a tangential fan 200without cylindrical symmetry, in accordance with an embodiment of thepresent invention. Tangential fan 200 is partitioned longitudinally intoa plurality of sections 210 each containing blade members 214 spacedsubstantially evenly about the circumferences 220 of hub members 212.Blade members 214 of all sections 210 are aligned longitudinallyparallel with a longitudinal rotation axis 222 of tangential fan 200concentric with circumferences 220. However, blade members 214 of asection 210 are offset by a circumferential angle φ relative to blademembers 214 of adjacent sections 210. This configuration breaks thetraditional cylindrical symmetry and minimizes in-phase reflection ofshock energy that causes laser pulse output energy fluctuations.

For example, tangential fan 200 as shown in FIG. 2a is partitioned into18 longitudinal sections 210 (shown only in part for clarity). Eachsection 210 includes 23 blade members evenly spaced about circumference220. A number of different circumferential offset schemes can be used.In accordance with one such scheme, circumferential offset angle φbetween adjacent sections 210 is equal to 1/18 of a full circumferentialrevolution of 360 degrees, such that a full 360-degree circumferentialrevolution of the blade pattern is evenly distributed among the 18sections 210. According to another such scheme, a circumferential offsetangle φ of 1/18 times 1/23 of a full circumferential revolution isapplied between adjacent sections 210, thereby evenly distributing acumulative circumferential offset of one blade position among the 18sections 210. According to other such schemes, the circumferentialoffset angle φ between blade positions of adjacent sections 210 can bean integral multiple of 1/18 or of 1/18 times 1/23 of a fullcircumferential revolution.

In some configurations, circumferential offset angle φ between adjacentsections 210 shifts in a constant circumferential direction from end toend of tangential fan 200. In other configurations, circumferentialoffset angle φ between adjacent sections 210 reverses direction one ormore places in progressing from end to end of tangential fan 200. FIG.2b is a graphical representation of different examples 241-252 of thedependence of circumferential offset angle φ on section 210 oftangential fan 200 having 18 sections 210. Sections 210 of tangentialfan 200 are displayed sequentially along the horizontal direction.Circumferential offset angles φ are shown by the vertical positions ofhorizontal line segments. Example 241 illustrates a conventionaltangential fan configuration having no offset of blade patterns betweensections. Example 247 illustrates a constant circumferential shiftdirection. Other examples, 242-246 and 248-252, illustrate varyingcircumferential shift configurations.

Of importance, circumferential offset angle φ should not be an integralmultiple of the circumferential spacing between adjacent blade members214 within a section 210. Such a circumferential offset angle φ wouldresult in a replication of the original blade pattern orientation, andthus would not break the traditional cylindrical symmetry. Although theprevious examples describe an equal number of blade members 214 persection 210, the number of blade members in a section can vary fromsection to section.

Referring to FIG. 2a, tangential fan 200 breaks the traditionalcylindrical symmetry by circumferentially offsetting the circumferentialorientations of blade members 214 at differing longitudinal positionsalong the length of tangential fan 200. When tangential fan 200 isinstalled in laser chamber 100 (see FIGS. 1a-1b), such circumferentialoffsets provide a differing reflection angle of shock energy for eachdiffering blade member orientation. The tangential fan configurationsdescribed in connection with FIG. 2a replace each traditionally straightcontinuous blade member 144 of tangential fan 140 (see FIGS. 1a-1b) witha plurality of parallel but offset blade members 214 having adistribution of differing circumferential orientations. In the examplesdescribed above, the reflection of shock energy at a particular angle byany individual blade member 214 of tangential fan 200 is approximately18 times smaller than the reflection of shock energy at that same angleby a longer individual blade member 144 of conventional tangential fan140. As a result, a substantially smooth temporal distribution ofreflected shock energy reenters discharge region 122 (see FIGS. 1a-1b)as tangential fan 200 rotates, greatly reducing laser pulse outputenergy fluctuations.

Additionally, the circumferential offset of blade members reduces theaerodynamic buffeting effect described in connection with FIGS. 1a-1b.Since each straight continuous blade member 144 of conventionaltangential fan 140 is replaced by a circumferentially offsetdistribution of shorter blade members 214 of tangential fan 200, only amuch smaller blade member 214 passes in close proximity to the edge ofanode assembly 120 at a given time. Accordingly, each time such asmaller blade member 214 passes the edge of anode assembly 120, acorrespondingly smaller gas pressure fluctuation is generated. This inturn reduces the buffeting vibration amplitude transmitted to thetangential fan bearings.

For example, if each blade member 214 extends only 1/18 of the length oftangential fan 200, then it generates only approximately 1/18 of the gaspressure fluctuation of a full-length blade member 144 of conventionaltangential fan 140. This reduced gas pressure fluctuation in turntransmits to the tangential fan bearings only approximately 1/18 of thevibration amplitude transmitted with conventional tangential fan 140.

FIG. 2c is a schematic view of a skewed or twisted substantially helicalblade fan structure 260. Tangential fans with skewed or twisted bladegeometry are commercially available, e.g., from Hi-Tech Blowers, Inc.,of 525 Northern Blvd., Great Neck N.Y. 11021. Twisted blade fanstructure 260 is effectively the continuous limit of tangential fan 200,wherein each blade member 264 is partitioned into an infinite number ofinfinitesimally short sections, having continuous substantially helicalrelative offset about a rotation axis 270 rather than a steppedcircumferential relative offset. Twisted blade fan structure 260 caninclude full-length blade members 264 as shown in FIG. 2c, having eithera clockwise or counterclockwise helical twist. Alternatively, twistedblade fan structure 260 can include multiple longitudinal sections ofblade members 264 having alternately reversed adjacent clockwise andcounterclockwise substantially helical twists, to cancel longitudinalaerodynamic effects. Twisted blade fan structure 260 typically includeshub members 262 to support and stiffen blade members 264. Twisted bladefan structure 260 also includes end flanges 266 to provide attachment torotating end shafts (described below in connection with FIGS. 4a-4e).

Relative to tangential fan 200, twisted blade fan structure 260 providesequal or greater reduction of laser pulse output energy fluctuations andof vibrations due to aerodynamic buffeting. However, twisted blade fanstructure 260 is considered to be more difficult to fabricate than istangential fan 200.

A further configuration for reducing pulse output fluctuations andaerodynamic buffeting is a tilted tangential fan axis (not shown) withinlaser chamber 100. Tangential fan rotation axis 142 (see FIGS. 1a-1b) istilted relative to the longitudinal axis of discharge region 122 andanode assembly 120. Vane members 152, 160 and anode assembly 120 areappropriately reconfigured. Performance benefits are expected from sucha tilted axis fan configuration. However, accommodating a tilted axisfan into a laser chamber 100 and making required structuralmodifications is judged to be impractically complex and expensive.Alternatively, a tangential fan having a variable or taperedcircumference can be employed. However, such a tangential fan wouldnecessitate substantial structural modifications to the laser chamber.

Similarly, reconfiguring the anode assembly and/or vane members alonecan achieve successful reduction in pulse output fluctuations andaerodynamic buffeting. FIGS. 2d and 2e are schematic end and front viewsrespectively of a tangential fan and anode assembly 280 having astationary tapered anode support bar or cutoff 282. The diagonal edges284 of cutoff 282 are located in close proximity to the outercircumference of blade members 144 of tangential fan 140, which rotatesabout rotation axis 142. Tapered edges 284 are oriented diagonallyrelative to rotation axis 142.

As tangential fan 140 rotates in a direction indicated by an arrow 286,blade member 144 passes gradually into close proximity to diagonal edge284 of cutoff 282. As illustrated in FIGS. 2d-2e, the portions of blademember 144 nearest the ends 288a, 288b of tangential fan 140 rotatefirst into close proximity with diagonal edges 284. Eventually theportions of blade member 144 midway between ends 288a, 288b rotate intoclose proximity with diagonal edges 284. This configuration effectivelybreaks the cylindrical symmetry of the conventional tangential fan andcutoff assembly illustrated in FIG. 1a, 1b. For example, tapered cutoff282 in combination with straight longitudinal blade members 144 ofconventional tangential fan structure 140, provides a reduction inaerodynamic buffeting equivalent to that of a straight conventionalcutoff or anode assembly 120 in combination with tangential fanstructure 200 or 260, having respectively staggered or twisted blademembers.

Alternatively, a tapered cutoff is configured such that portions ofblade member 144 midway between ends 288a, 288b rotate first andportions of blade member 144 nearest ends 288a, 288b rotate last intoclose proximity with the diagonal edges of the cutoff. In anotheralternative configuration, a tapered cutoff has an edge orientedsubstantially in a single direction, such that portions of blade member144 nearest one end, e.g., 288a, rotate first and portions of blademember 144 nearest the opposite end, e.g., 288b, rotate last into closeproximity with the diagonal edge of the cutoff.

Blade members 144, as illustrated in FIGS. 2d and 2e, are alignedsubstantially parallel to rotation axis 142. However, blade members 144need not be aligned precisely parallel to the rotation axis 142, but canalso be oriented on a slight diagonal relative to rotation axis 142, soas to be substantially parallel.

The above-described tangential fan configurations 200 and 260 can befabricated as brazed assemblies of individually stamped and formed bladeand hub members, similar to conventional tangential fan 140, asdescribed above in connection with FIGS. 1a-1b.

Alternatively, tangential fan configurations 200, 260 can be fabricatedas monolithic castings, which are then post-machined on their endflanges 266 (see FIG. 2c) inside and outside diameters, and thenelectroless nickel coated. A conventional die casting or investmentcasting process can be employed (see for example "Metal QualityStandards," Investment Casting Institute, 8350 N. Central Expressway#M1110, Dallas, Tex. 75206-1602, 1987), using an aluminum oraluminum-bronze casting alloy. Suitable aluminum alloys includelow-silicon alloys containing 3.5-6.5 per cent copper, 0-2.5 per centnickel or 0-1.5 per cent silver, and lesser concentrations of magnesium,titanium, iron, manganese, and various trace metals.

A tangential fan assembly can also be cast in sections, which are thenjoined together longitudinally. FIG. 3a is an isometric view of a castsection 310 of a tangential fan assembly, in an embodiment of thepresent invention. Cast section 310 includes integrally cast hub 312 andblade 314 members. An arrow designated by reference numeral 318 showsthe direction of fan rotation about an axis 332 relative to thecurvature of blade members 314. Conventional die casting or investmentcasting is performed using an aluminum or aluminum-bronze casting alloy,as described above in connection with FIGS. 2a and 2c, in order to meetrequired precise dimensional and alignment tolerances, as describedbelow in connection with FIGS. 4a-4e.

FIG. 3b is an isometric view of a portion of a tangential fan structure340, including a plurality of sections 310 joined togetherlongitudinally along a common axis 332. Typically, electron beam (EB)welding is employed for joining sections 310. The cast sections 310 areprecisely post-machined on their inside diameters, outside diameters,and on the end surfaces of hub members that mate with adjacent sections310 to which they will be joined. Sections 310 plus two end flanges,such as end flange 266 (see FIG. 2c) are then secured to a mandrel orother appropriate fixturing, to provide a mechanically straight andbalanced welded tangential fan assembly, and are EB welded around theentire outside diameter 316 between all mating sections 310 and endflanges. The welded final assembly is post-machined to achieve desiredfinal dimensions and tolerances, as described below in connection withFIGS. 4a-4e, and is then electroless nickel coated for corrosionresistance. For simplicity, blade members 314 of tangential fanstructure 340 are shown as longitudinally straight and aligned parallelbetween sections 310. In some embodiments, blade members 314 oftangential fan structure 340 are twisted, as shown in FIG. 2c, and/orare offset circumferentially from section to section, as shown in FIG.2a. Such an offset can be achieved by rotating sections 310 relative toone another about rotation axis 332 prior to welding.

A further method of fabricating a tangential fan assembly involvesmachining an entire fan assembly as a monolithic unit from a singleblock of material. FIGS. 4a and 4b are respectively longitudinal and endviews of a monolithic tangential fan structure 400 machined from asingle block of 6061 aluminum alloy, in accordance with an embodiment ofthe present invention. Machined tangential fan structure 400 ispartitioned into sections 410 between consecutive integral substantiallyannular hub members 412 and two integral substantially annular endflanges 416a and 416b disposed substantially concentrically about arotation axis 440. Integral blade members 414 extend longitudinallyacross each section 410 between consecutive hub members 412 and/or endflanges 416a and 416b. For simplicity, blade members 414 of machinedtangential fan structure 400 are shown as longitudinally straight andaligned parallel to rotation axis 440 between sections 410. In someembodiments, blade members 414 of tangential fan structure 400 aretwisted, as shown in FIG. 2c, and/or are offset circumferentially fromsection to section, as shown in FIG. 2a.

Hub members 412 provide structural rigidity to blade members 414 andthereby to machined tangential fan structure 400. Particularly,tangential fan structure 400 is machined such that stiffening fillets418 are formed in the corners connecting the ends of blade members 414and the annular surfaces of hub members 412 and end flanges 416a, 416b.FIG. 4c is an isometric detail view showing fillets 418 having radii of,e.g. 2.54 mm, connecting blade members 414 with a hub member 412.

The outside diameter 420 of tangential fan structure 400 is typicallymachined within a tolerance of ±0.13 mm. Counterbores 422a and 422bmachined in respective end flanges 416a, 416b are flat and parallelrelative to one another within typically ±0.013 mm, and are concentricwith an inside diameter 424 within ±0.051 mm. Bolt holes 426 are drilledin end flanges 416a, 416b, for example equally spaced eight places on abolt circle 428, for attaching rotating shaft assemblies (shown below inFIGS. 4d-4e). An extra clocking hole 430 is provided on bolt circle 428to accommodate a pin (not shown) that restricts the drive shaft assemblyto one end only of tangential fan structure 400. Dimensions andtolerances applying to machined tangential fan structure 400 can applyas well to all other tangential fan structures 200, 260, 340 using anyof the methods of fabrication described above.

FIGS. 4d and 4e are an end view and a crosssectional side view,respectively, of a rotating shaft assembly 450 configured to attach totangential fan structure 400, in accordance with some embodiments. Inother embodiments, a rotating shaft assembly similar to rotating shaftassembly 450 can attach to tangential fan structures such as tangentialstructures 140, 200, 260, and 340. A circular shoulder 452 fitsconcentrically into counterbore 422a, 422b to locate rotating shaftassembly 450 accurately relative to tangential fan assembly 400. Boltholes 454 on a concentric bolt circle contain bolts (not shown) tosecure rotating shaft assembly 450 to end flange 416a, 416b oftangential fan assembly 400. An extra clocking hole 456 is included todifferentiate the drive side from the idle side of tangential fanassembly 400. An outer shaft diameter 460 is tapered stepwise to aconcentric bearing shaft 462, configured to rotate within a shaftbearing (not shown).

The machined tangential fan structure 400 is electropolished to achievea surface finish of the order of 0.4 μm to 0.6 μm (15 microinch to 25microinch) Ra on all surfaces, and is then electroless nickel coated.

Monolithic machined, cast, or cast-and-welded tangential fans, such asthose described above in connection with FIGS. 2a-2c, 3b, and 4a-4b alsoallow the shaping of airfoil blade members, which are difficult orimpractical to produce using conventional stamping processes. Forexample, FIG. 3c is a schematic cross-sectional view of an airfoil blademember 320 viewed along direction C--C of FIG. 3a. Airfoil blade member320 is shown superimposed in cross-section on a circumference 330 of atangential fan structure, e.g., tangential fan structures 200, 260, 340,400. Conventional blade members 314 are shown in cross-section forcomparison. For clarity, only one airfoil blade member 320 and twoconventional blade members 314 are shown. An arrow designated withreference number 318 indicates the rotation direction of the tangentialfan about axis 332 relative to the curvature of the blade members.Whereas conventional blade members 314 are substantially uniform inthickness, airfoil blade member 320 typically has a "tear-drop" profile,including a rounded leading edge 322, a thickened midsection 324, and atapered trailing edge 326. Incorporation of airfoil blade members 320 isexpected to improve the aerodynamic efficiency of tangential fans, suchas those described in connection with FIGS. 2a-2c, 3b, and 4a-4b.

FIG. 4f is a cross-sectional detail view of machined tangential fanstructure 400 taken along direction F--F of FIG. 4a. Blade members 414are shown superimposed on annular hub member 412 having outsidecircumference 420 and inside diameter 424, disposed concentrically aboutrotation axis 440. In some embodiments, inside diameters 424 of hubmember 412 and blade members 414 are machined such that the insidediameter defined by the plurality of blade members 414 coincides withthe inside diameter 424 of substantially annular hub member 412.

The above-described fabrication methods, including monolithic machining,monolithic casting, and welded casting, can also be applied to othertangential fan configurations, such as above-described conventionaltangential fan 140.

FIG. 5 is a graphical representation of the dependence of relative laseroutput energy of an excimer laser on concentration of commoncontaminants. Relative output energy is displayed along the verticalaxis and concentration in parts per million (ppm) is displayed along thehorizontal axis. A concentration of approximately 60 ppm of silicontetrafluoride SiF₄ in lasing gas 108 can reduce laser pulse outputenergy by approximately 8-12 per cent. It has been discovered that brazematerial typically containing approximately 13 per cent silicon inconventional tangential fan 140 has been a major source of siliconcontamination in current excimer laser systems. Silicon (Si) reacts withfluorine (F₂) in the lasing gas 108 to form SiF₄. Electroless nickelcoating is a somewhat porous and imperfect seal and develops microcracksover time, thereby exposing the underlying Si to the reactive fluorinegas mixture. A lasing gas fill, which typically has a working life ofseveral days, starts with a low SiF₄ concentration, typically less than0.3 ppm. With a conventional brazed tangential fan, this level rises toa range of about 15-18 ppm after a period of three days.

Cast, cast-and-welded, or monolithic machined tangential fans, such asthose described above in connection with FIGS. 2a-2c, 3b, and 4a-4brequire no braze material or other added contaminant-forming materialduring fabrication and thus are essentially free from Si, greatlyreducing the potential adverse effect on laser pulse output energy.Tests performed on a monolithic machined tangential fan 400 provided bythe present invention have shown a SiF₄ concentration of less than 3 ppmafter three days. Cast or cast-and-welded tangential fans 200, 260, 340provided by the present invention are expected to perform comparably.However, a monolithic machined tangential fan 400 has a polycrystallinealloy structure typical of wrought aluminum, which has lower porosityand therefore avails itself better to sealing with nickel coatingrelative to cast aluminum alloys. Thus, monolithic machined tangentialfan 400 is expected to contribute lower contamination relative to castor cast-and-welded tangential fans 200, 260, 340. In addition,cast-and-welded fan 340 has large-area flat surfaces inside narrowcrevices between adjacent sections 310, that are difficult to clean andevacuate for processing and operation.

In conventional tangential fan 140, significant stiffness againstbending moments is provided by hub members 146, without which blademembers 144 would be too flexible to be useful above approximately 100rpm. Hub members 212, 312, 412 of tangential fans 200, 260, 340, 400 inaccordance with embodiments of the present invention, also contributesignificant structural stiffness. Additionally, monolithic machined,cast, and cast-and-welded tangential fan structures 200, 260, 340, 400provide greater stiffness relative to conventional brazed tangential fanassembly 140 having individually stamped and formed blade 144 and hub146 members. Particularly, monolithic tangential fan structure 400provides precise control of the radiused connections from hub member 412to blade member 414 and from end flange 416a, 416b to blade member 414.This controlled fillet 418, which adds stiffness, also results in higherbending mode frequencies and correspondingly higher permitted operatingspeeds, ranging as high as 5,000 or more rpm. The greater stiffness ofmonolithic machined tangential fan structure 400 also increases theability of such a tangential fan to resist "aerodynamic buffeting"effects within laser chamber 100.

In particular, embodiments of the invention provide for increasing fanstiffness by optimally selecting the number and longitudinal placementof hub members, such as hub members 146, 212, 312, and 412, in thetangential fan structure. Investigations have shown that the naturalfrequency of the first bending vibration mode increases as the number ofhub members increases. Severe vibrations of the tangential fan occur atrotation rates approximating one-half of the natural frequency of thefirst bending mode, because the fan structure distorts laterally at eachhalf period of rotation. Whereas in structures typical of previoustangential fans, this limits rotation frequencies to less than a fewhundred rpm, optimal selection of the number and longitudinal placementof hub members extends the range of rotation frequencies to 5,000 ormore rpm. Therefore, increasing the natural frequency of the firstbending mode by optimal selection of the number and longitudinalplacement of hub members improves the performance of a tangential fan.

The surface and dimensional control of machined monolithic tangentialfan 400 are expected to be more precise than those of a brazed or casttangential fan. Monolithic machined tangential fan 400 should requireonly a minimum of balancing and should impart a smoother flow of gas dueto blade-to-blade uniformity.

The present invention provides a tangential fan and cutoff assembly thatis economical to fabricate and has sufficient mechanical rigidity toallow balancing and post-machining for precision tolerances, and tominimize vibration during operation. Embodiments of the invention reduceaerodynamic buffeting effects and laser pulse output energyfluctuations, and allow shaping of efficient airfoil blade members.Additionally, a tangential fan provided by the present invention doesnot introduce silicon contamination into the laser chamber. Such a fanis configured to recirculate lasing gas in a laser chamber, particularlyan excimer laser chamber, and more particularly a laser chamber of akrypton fluoride or argon fluoride excimer laser, or of a fluorine (F₂)molecular gas laser.

While embodiments of the present invention have been shown anddescribed, changes and modifications to these illustrative embodimentscan be made without departing from the present invention in its broaderaspects. Thus it should be evident that there are other embodiments ofthis invention which, while not expressly described above, are withinthe scope of the present invention. Therefore, it will be understoodthat the appended claims necessarily encompass all such changes andmodifications as fall within the described invention's true scope; andfurther that this scope is not limited merely to the illustrativeembodiments presented to demonstrate that scope.

What is claimed is:
 1. An electric discharge laser apparatuscomprising:a tangential fan configured to recirculate a lasing gasmixture, said tangential fan comprising; a rotation axis having a firstend and a second end opposite said first end; a circumferencesubstantially concentric with said rotation axis; a plurality of blademembers disposed proximate to said circumference, such that a blademember extends substantially parallel to said rotation axis between saidfirst end and said second end; and a cutoff member, said cutoff memberbeing fixed in position and having a tapered edge proximate to a portionof said circumference, such that said tapered edge is orienteddiagonally relative to said rotation axis and said plurality of blademembers; wherein said tapered edge oriented diagonally relative to saidblade members reduces pulse output fluctuations and aerodynamicbuffeting as compared with cutoff members which are parallel to fanblade members.
 2. The apparatus of claim 1, wherein said tapered edgecomprises two edge segments, said two edge segments being orienteddiagonally relative to one another.
 3. The apparatus of claim 1, whereinthe number of said blade members is an odd integer.
 4. The apparatus ofclaim 1, wherein said blade member has an airfoil cross-sectional shape.5. The apparatus of claim 1, further comprising a plurality of hubmembers attached to said blade members, said hub members being disposedsubstantially transversely relative to said rotation axis, wherein thenumber and axial placement of said hub members are selected such thatthe natural frequency of bending mode vibration of said tangential fanis controlled.
 6. The apparatus of claim 5, wherein said hub members areselected such that the natural frequency of bending mode vibration ofsaid tangential fan is greater than twice the rotation frequency of saidtangential fan.
 7. The apparatus of claim 1, wherein the material ofsaid tangential fan is an alloy selected from the group consisting of6061 aluminum, an aluminum alloy containing additive metals consistingsubstantially of 3.5-6.5 per cent copper and 0-2.5 per cent nickel, andan aluminum alloy containing additive metals consisting substantially of3.5-6.5 per cent copper and 0-1.5 per cent silver.
 8. The apparatus ofclaim 1, wherein said electric discharge laser apparatus furthercomprises a transverse excited molecular laser.
 9. The apparatus ofclaim 8, wherein said molecular laser is selected from the groupconsisting of argon fluoride excimer lasers, krypton fluoride excimerlasers, and fluorine (F₂) molecular lasers.